A constant need exists for improved means for inspecting printed circuit boards for missing components. The prior art includes a number of investigations directed to the use of a correlation process employing both digital, as well as optical, methods. However, these have met with limited success.
Straightforward digital techniques have also been examined. The main limitations have usually centered upon the degree of success in examining a circuit board in its entirety.
Two conventional techniques involving interferometry are known. The first has been used for many years to inspect such items as lenses. The technique utilizes a monochromatic beam (initially filtered light--later laser light) which is sent through the item to be inspected and combined with an uninterrupted beam. The path difference of the uninterrupted beam has phase changes in it so that, when combined, an anomaly becomes evident in a developed fringe pattern which is usually a set of parallel lines. A modification of this technique is to have one beam reflect from an opaque item (which could be a circuit board) and then combining the incident and reflected beams.
A second conventional technique projects a Moire pattern upon an item to be inspected and the reflected pattern is examined. The Moire effect is one in which two nearly identical sets of lines are perceived to interact causing deviations in an array of fringe lines for the field of lines where anomalies occur. This effect can be readily seen by viewing any TV picture through something like an ordinary screen, or looking at the screen when certain lined pictures are viewed. A modification of this approach could employ circular rings instead of parallel lines. Sets of sinusoidal waves, etc., could also be used.